A wide variety of intrusion detection equipment is known. Some of it is for use in perimeter security systems where for example, a special taut wire perimeter fence may be installed which, when cut or jarred, generates a signal which in turn triggers an alarm. Enclosed buildings or spaces within such buildings often have intrusion detection sensors for detecting the opening of windows or doors, the cutting of electric circuits, etc. and may be provided with infra-red or other movement-detecting sensors within confined areas. Sensors are known for responding to seismic vibrations; platform sensors are available for placement around railway tracks to sense unwanted human intrusion upon such tracks.
Conventionally, the signal processing circuitry following the particular intrusion detection sensor employed will be tailored to the specific sensor and specific adaptation at hand.
If the intrusion detection system designer perceives that more than one type of intrusion is expected, then several detectors may be utilized, each with its own associated circuitry. In some cases, the outputs of two or more such circuits are compared against some standard or threshold, as for example in U.S. Pat. No. 4,223,304 (Barowitz, Sep. 16, 1980) and U.S. Pat. No. 4,107,660 (Chleboun, Aug. 15, 1978).
A difficulty with the known intrusion detection signal processors is that they are relatively inflexible, being adapted for use with particular sensors operating within particular frequency ranges. However, in a particular application, a number of different types of intrusion may be expected, and consequently several different types of sensor may be required to be employed.
A further difficulty with many of the known systems is that they are sensitive to transients and spurious signals that may cause false alarms. To some extent the known systems have circumvented these problems by incorporating delay circuitry that rejects signals above a particular amplitude threshold but whose duration is too short to be likely to represent an intrusion. However, these systems tend not to be able to discriminate persistent signals of sufficient amplitude caused by unwanted intrusion from those caused by an increase in vibration level generally. By way of example, suppose that a secured military area within a perimeter fence attached to an intrusion detection system, is a site for frequent helicopter landings. If the sensitivity of the perimeter fence detector and associated circuitry are set at a high enough level to detect unwanted human intrusions, the vibration due to a helicopter landing may be sufficient to trigger an unwanted false alarm.